So the important principle here is to remember you are aiming for ~800 calories a day, and it is important to do this under medical or a dietitian reviewing you.
There are many ways to use meal replacement shakes like optislim, optifast and so on. But again make sure they are proper weight loss replacement shakes, not some cheap alternative that is not safe.
One way to do it is you go through an intensive phase for 2-4 weeks then a transition and maintenance phase for 1-3 months where you replace the low calorie shakes/meals that you bought with healthy food you and have cooked.
Intensive phase = all meal replacements for 2-4 weeks
Second phase = one proper healthy meal + 2 meal replacements for other meal times
Third phase = Two proper healthy meals and 1 meal replacement
Ongoing = Three healthy meals but can use meal replacements when desired/needed/convenience
Remember you can start at any phase depending on your routine.
Each shake is ~150 calories-200 calories, hence you can have up to 4/day. Do not go hungry! But this is meal REPLACEMENT, do not add to other meals. Hence if you come to a dinner time and you want a meal then have that instead of the shake, but if you have a shake then do not have a meal. It is one or the other… NOT BOTH!
Some people find they get bored of the flavours after a while so an easy way around this is to add in frozen berries or some mango or banana. But not too much as these things have a fair bit of calories themselves.